In general, electrochemical cells have a mono polar structure. Such a mono polar electrochemical cell includes a positive electrode composed of a positive active material formed on a current collector and a negative electrode composed of a negative active material formed on another current collector. These electrodes are disposed with opposite polarity sides facing each other, and a separator is inserted between the electrodes to form a unit-cell structure.
FIG. 1 illustrates a mono polar electrochemical cell of the related art.
Referring to FIG. 1, the electrochemical cell 10 of the related art includes a positive electrode 11, a negative electrode 12, a separator 13, an electrolyte 14, terminals 15-1 and 15-2, and a case 16. The illustrated electrochemical cell is the minimum basic operation unit which is called a unit cell.
Electric energy is stored in the positive electrode 11 and the negative electrode 12.
The separator 13 inserted between the positive and negative electrodes 11 and 12 is electrically nonconductive. However, the separator 13 may be omitted if the positive and negative electrodes 11 and 12 can be not in contact with each other without the separator 13. In a recent lithium polymer battery, solid polymer electrolyte is used instead of a separator; however, the solid polymer electrolyte contains a liquid electrolyte, and electrochemical reactions are produced by ions contained in the liquid electrolyte. That is, basically, the lithium battery is not different from a battery using a separator and a liquid electrolyte.
The separator 13 is formed of a material capable of transmitting the electrolyte 14, such as porous polymer, fiber glass mat, and paper.
The operational voltage of such electrochemical unit cells having the above-described structure is only several volts. Among electrochemical cell batteries, a lithium ion battery has a relatively high operational voltage; however, the nominal voltage of the unit cells of the lithium ion battery is also low at about 3.6 Volts.
Therefore, electrochemical cells should be connected in series for being used in application fields such as industrial and vehicle application fields requiring several tens to several hundreds of volts, as shown by unit cells 21, 22, and 23 in FIG. 2.
Since the unit cells 21, 22, and 23 are connected in series, the assembled structure and assembling processes are complicated, and additional parts such as bus bars and screws are necessary. Furthermore, the volume, weight, and resistance of the assembled structure are increased. As shown in FIG. 2, bus bars are used to connect neighboring unit cells, and screws are used to fix the bus bars to the unit cells.
An electrochemical cell 30 having a bipolar structure as shown in FIG. 3 has been developed to address the above-described limitation.
In the electrochemical cell 30 having a bipolar structure, electrodes are configured such that both sides of current collectors 31 have opposite polarities and electrodes having opposite polarities face each other with a separator 32 being disposed therebetween. The lowermost electrode is composed of an active material layer formed on one side of the lowermost current collector 31, and the uppermost electrode is composed of an active material layer formed on one side of the uppermost current collector 31.
In manufacturing electrodes of the bipolar electrochemical cell 30, if positive and negative electrodes are formed on the same material of the current collector 31, a positive active material layer 33 and a negative active material layer 34 are formed on both sides of the current collector 31 having a sheet shape. If positive and negative electrodes have to be formed on different materials of the current collector 31, a complex current collector having a laminated structure formed of different materials is used as the current collector 31. In FIG. 3, reference numeral 35 denotes gaskets, and reference numerals 36 and 37 denote terminals. The gaskets 35 are used as electrolyte sealing and isolating members for sealing unit cells, such that undesired phenomena such as current leakage, side reactions, corrosion caused by the side reactions can be prevented between unit cells.
Generally, a current collector of a positive electrode of a lithium ion battery is formed of aluminum, and a current collector of a negative electrode is formed of copper. In a lithium ion battery having a bipolar structure, current collectors having a multi-layer structure composed of aluminum and copper lamination sheets may be used. In a general electrochemical cell having a bipolar structure, an electrolyte isolation member is installed on an edge portion of an electrode so as to prevent undesired phenomena between unit cells, such as current leakage, side reactions, and corrosion caused by the side reactions. For the same reason, an electrolyte should not be transmitted through a current collector of an electrode in the electrochemical cell having a bipolar structure.
In the bipolar structure, if electrolytes of neighboring unit cells are not securely isolated, current leakage occurs between the unit cells, and the unit cells corrode easily. Therefore, it is very difficult to isolate electrolytes of neighboring unit cells securely for a long time under various operation environments.
Another limitation of a bipolar electrochemical cell is that it is difficult to manufacture a high-capacity bipolar electrochemical cell. The areas of electrodes should be increased to increase the capacity of a bipolar electrochemical cell; however, in this case, the structural strength of the bipolar electrochemical cell is reduced, and it is more difficult to isolate electrolytes of neighboring unit cells and inject electrolyte into the unit cells. Furthermore, it is troublesome to assemble electrodes and separators into an electrochemical cell after electrolyte is filled between the electrodes and the separators.
An electrochemical cell having a quasi-bipolar structure similar to the bipolar structure has been developed.
FIG. 4 is a cross-sectional view illustrating a quasi-bipolar electrochemical cell of the related art.
Referring to FIG. 4, the quasi-bipolar electrochemical cell 40 includes current collectors 41, separators 42, positive active material layers 43, negative active material layers 44 and 45, and gaskets 46.
In the above-described bipolar electrochemical cell, active material layers having opposite polarities are disposed on both sides of a current collector. However, as shown in FIG. 5, a quasi-bipolar electrochemical cell electrode 50 includes a mono polar electrode and a quasi-bipolar electrode. The mono polar electrode includes current collectors 51 and 52, and positive and negative active material layers 53 and 54 respectively disposed on the current collectors 51 and 52 and connected to a terminal. The quasi-bipolar electrode includes a current collector 56, and positive and negative active material layers 57 and 58 disposed on the current collector 56 and spaced apart from each other with a current collector extension part 55 being located therebetween.
The electrodes are disposed with opposite polarities facing each other, and separators are disposed between the electrodes. In the quasi-bipolar structure, the quasi-bipolar electrode is used as opposite electrodes of neighboring unit cells. That is, neighboring unit cells are connected in series to each other through the current collector extension part of the quasi-bipolar electrode. In a bipolar structure, a current flows in a direction perpendicular to electrodes; however, in a quasi-bipolar structure, a current flows in a direction parallel to electrodes, that is, in a direction parallel to current collectors. In a quasi-bipolar electrochemical cell, an electrolyte isolation member, such as a gasket and an adhesive that are formed of a nonconductive material through which electrolyte cannot be transmitted, is disposed on a current collector extension part of a quasi-bipolar electrode located at the center portion of the quasi-bipolar electrode so as to isolate electrolytes of neighboring unit cells. However, if there is no extra electrolyte except for electrolyte at an active material layer of an electrode and a separator, such an electrolyte isolation member is not always necessary. In a sealed recombination lead acid battery, extra electrolyte does not exist at other regions than an active material layer of an electrode and a separator, and although extra electrolyte may exist, the extra electrolyte evaporates by an electrochemical reaction. Therefore, in a certain case, an electrolyte isolation member may be not necessary.
FIG. 5 is a perspective view illustrating an electrode of an electrochemical cell having a quasi-bipolar structure according to the related art.
As shown in FIG. 5, the quasi-bipolar electrochemical cell electrode 50 includes a mono polar electrode and a quasi-bipolar electrode. The mono polar electrode includes current collectors 51 and 52, and positive and negative active material layers 53 and 54 respectively disposed on the current collectors 51 and 52. The quasi-bipolar electrode includes a current collector 56 having a connection region 55, and positive and negative active material layers 57 and 58 symmetrically disposed with respect to the connection region 55.
FIG. 6 is a cross-sectional view illustrating an electrochemical cell having a stacked quasi-bipolar structure in the related art.
As shown in FIG. 6, the electrochemical cell 60 having a stacked quasi-bipolar structure includes negative active material layers 61 and 62, separators 63, positive active material layers 64, current collectors 65, and gaskets 66.
FIG. 7 is a view for explaining voltage variations of an electrochemical cell having a stacked quasi-bipolar structure in the related art.
In FIG. 7, reference numerals 71, 72, 73, and 74 denote current collectors, an electrolyte isolation wall, active material layers, and separators, respectively. The capacitance of one of the active material layers is 2C+Δ, and the capacitance of the others is 2C.
In manufacturing electrodes of an electrochemical cell having a quasi-bipolar structure, if the same material of a current collector is used for positive and negative electrodes, electrodes are formed by a generally used active material forming method using a sheet, mesh, or grid current collector; however, if different current collector materials are used for positive and negative electrodes, after positive and negative electrodes are formed in a manner such that an active material does not exist at edge portions of a current collector, the portions where an active material does not exist may be electrically connected by an electric connecting method such as welding so as to form electrodes. Generally, the surface of a current collector is treated like an etched aluminum foil to increase the surface area of the current collector so as to attach an active material layer to the surface of the current collector more reliably.
In a quasi-bipolar structure, a quasi-bipolar electrode and a mono polar electrode may be wound around a core and connected in series to each other along the longitudinal direction of the core. This structure is advantageous for a high-capacity electrochemical cell; however, there is a need for an improved method of forming a reliable electrolyte isolation barrier wall between an electrode assembly and a case. In addition, to increase the reliability of products, what is needed is an electric connection method for connecting a voltage equalizing circuit to electrodes of unit cells so as to prevent voltage variations across the unit cells and current leakage of the unit cells caused by deficiency in electrolyte isolation ability of bipolar and quasi-bipolar structures.